The invention relates to polybenzazole (PBZ) polymers.
Polybenzazole (PBZ) polymers are a known class of polymers comprising polybenzoxazole (PBO), polybenzothiazole (PBT), polybenzimidazole (PBI) and copolymers of these polymers. Polybenzazole polymers ordinarily contain a plurality of mer units chosen from AB-PBZ mer units, which typically comprise:
(1) an aromatic group (Ar); and PA0 (2) an azole ring fused with the aromatic group, PA0 ( 1 ) an aromatic group (Ar); PA0 (2) a first and a second azole ring fused to the aromatic group; and PA0 (3) a divalent organic moiety (DM), which is inert and stable under polybenzazole polymerization conditions, bonded to the 2-carbon of the first azole ring. PA0 DM is a bond or a divalent organic moiety which is stable and inert in acid under polymerization conditions, and PA0 each Z is independently an oxygen atom, a sulfur atom, or a nitrogen atom that is bonded to a hydrogen atom or an organic moiety which is stable and inert in acid under polymerization conditions. PA0 (1) each nitrogen atom is ortho to the Z moiety in its azole ring; and PA0 (2) if the mer unit has two azole rings, the nitrogen atom and Z moiety of one azole ring may be in either cis position or trans position with respect to the nitrogen atom and Z moiety of the other azole ring, for example, as illustrated in Formulae 2(a) and (b). ##STR2## The same understandings apply with respect to amine groups and Z moieties in o-amino-basic moieties bonded to aromatic moieties. Cis and trans isomerism of PBZ polymers is illustrated in 11 Ency. Poly. Sci. & Eng., supra, at 602, which is incorporated herein by reference.) PA0 (1) a first aromatic group (Ar.sup.1): PA0 (2) two electron-deficient carbon groups (Q) bonded to said first aromatic group; PA0 (3) a poly(aromatic sulfide) block bonded to the first aromatic group, said poly(aromatic sulfide) block having a plurality of mer units, which each contain: PA0 (a) an aromatic group (Ar.sup.s); and PA0 (b) a sulfur atom linking said aromatic group (Ar.sup.s) either to an aromatic group (Ar.sup.1) in the polybenzazole block or to an aromatic group (Ar.sup.s) in an adjacent aromatic sulfide mer unit.
and AA/BB-mer units, which typically comprise:
Mer units are usually linked by a single bond from an azole ring in one mer unit to an aromatic group (Ar) or divalent organic moiety (DM) in an adjacent mer unit. The divalent organic moieties (DM) may be aliphatic, or even in some cases may be a bond, but are usually aromatic groups. AB-PBZ and AA/BB-PBZ mer units usually comply with formulae 1(a) or (b). ##STR1## wherein: each Ar is an aromatic group,
(For the purpose of this application, when the nitrogen atoms and Z moieties of an azole ring are depicted as bonded to an aromatic group without indicating their position, as in Formulae 1(a)-(b), it shall be understood that:
Polybenzazoles are synthesized by the reaction of one or more difunctional monomers, each of which contains at least two azole-forming moieties chosen from the group consisting of electron-deficient carbon groups and o-amino-basic moieties. Such reactions are illustrated in formulae 2(a) and (b). ##STR3## wherein each Q is an electron-deficient carbon group and all other moieties have the meaning and preferred embodiments previously given.
Polybenzazole polymers, their properties and their synthesis are discussed in detail in the following references: Sybert et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,772,678 (Sep. 20, 1988): Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,703,103 (Oct. 27, 1987): Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,533,692 (Aug. 6, 1985); Wolfe et al., Liquid Crystalline Poly(2,6-Benzothiazole) Compositions, Process and Products, U.S. Pat. No. 4,533,724 (Aug. 6, 1985); Wolfe, Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,533,693 (Aug. 6, 1985); Imai et al. "Polybenzoxazoles and Polybenzothiazoles," 83 Makromol. Chem. 167 (1965), Evers, Thermoxadatively Stable Articulated p-Benzobisoxazole and p-Benzobisthiazole Polymers, U.S. Pat. No. 4,359,567 (Nov. 16, 1982); Tsai et al., Method for Making Heterocyclic Block Copolymer, U.S. Pat. No. 4,578,432 (Mar. 25, 1986) and 11 Ency. Poly. Sci. & Eng., Polybenzothiazoles and Polybenzoxazoles, 601 (J. Wiley & Sons 1988), which are incorporated herein by reference.
Polybenzazoles are noted for their high tensile strength, their high tensile modulus, their resistance to most organic solvents, and their resistance to thermal degradation at moderate temperatures such as about 200.degree. C. However, thermal gravimetric analysis shows that PBZ polymers experience a serious loss of polymer weight during prolonged exposure to temperatures between about 250.degree. C. and about 400.degree. C., which limits their utility for use in very high temperature applications. Furthermore, polybenzazoles are soluble in many strong acids, such as methanesulfonic acid. Most importantly, the compressive strength of PBZ polymers is much lower than their tensile strength, and this seriously limits their utility in structural applications.
What are needed are new polybenzazole polymers or compositions, having improved stability in high temperatures, improved resistance to acids, and/or better compressive strength.